In the oldest methods of forming a continuous paper or bard web, which are still most commonly applied, forming of the web takes place in a horizontal so-called fourdrinier wire section, or a planar wire section. In addition, two main groups may be distinguished amount the wire sections, or former types, namely actual twin-wire formers and so called hybrid formers. In the actual twin-wire formers, forming of the web takes place from beginning to end between two wires. In the hybrid formers, the web is formed first onto one wire, after which this partly formed web is passed to a dewatering zone that is being formed between tow wires for final stabilization of the fibers.
One problem in the formers known from the prior art is rewetting of the web. For instance, when a suction box of high vacuum is used in a former, even after the box rewetting of the web still occurs, before the transfer of the web to the press section. On the other hand, it is known that wires at all times carry water in an amount of their own mass under normal running conditions, said water tending to move into the web if there is no force or other arrangement to prevent it.
Conventionally, vacuum zones, for instance in suction rolls, have been arranged to increase in suction, e.g. the first chamber of 25 kPa and the second chamber of 66 kPa in the case where attempts are made to increase the solids proportion without clogging up the web. A suction box of high vacuum usually arranged so that it is the last dewatering element in the former. It is known that solids may be increased by a high vacuum, but it is difficult to maintain the obtained solids.
With respect to the prior art, reference may be made to the magazine article of Szikla, Palokangas, Rolefelt in wetpressing, Paperija Puu 73, 1991, reporting an investigation of press felts in which it was observed that a vacuum of 20-40 kPa is enough to hold water in the felt after a nip, thereby minimizing rewetting in a press. The result has been obtained with modern felts, in which the resistance to flow is small.
With respect to prior art, reference may also be made to EP Patent Application No. 0 371 768, which discloses a web forming method and apparatus, said method being applied in a wire section of a paper machine or equivalent, in which the wire section is formed by a bottom wire loop with the main portion of its top run being substantially horizontal and a top wire loop cooperating therewith, and in which method a fiber suspension coming out of a headbox of the paper machine is supplied to the first part of the top run of said bottom wire loop, said part forming a first dewatering zone, after which the fiber layer from which part of the water has been removed is passed to a second dewatering zone in which said top wire loop covers the fiber layer, from which part of the water has been removed, such that the removal of the water from the fiber layer continues at least in two stages in the area of said second dewatering zone, after which the top wire loop is separated from the nearly formed fiber web, which is guided to follow the run of the bottom wire loop forwards for the following processing stages of the web. In connection with this prior-art arrangement a separation suction box has been used in the downwards slanting run of the bottom wire between a forming roll and a suction roll, the purpose of said suction box being to ensure that the web does not follow the wire, which returns to the wire section, but rather is transferred from the wire section to a press section. In this known arrangement of the web, supported by the bottom wire and ensured by the suction box, moves over the suction roll to the run between ti and the drive roll of the wire, from which the web is transferred, in a way known in itself, by means of a pick-up roll and a felt forwards to the press section of the paper machine. However, in this arrangement no attention has been paid to the prevention of rewetting.